detect need for "brew cleanup" in doctor

User will receive a warning if more than one copy of Homebrew-cask
is installed.

References: #4857
Merge pull request #4963 from rolandwalker/doctor_brew_cleanup_needed

detect need for "brew cleanup" in doctor
Annotatate unexpected $LOAD_PATH in doctor

Warning if first element is not Homebrew-cask's own lib dir